This invention relates to a caisson for sealing the spaces between the underside of the frame of an underwater structure and the bedrock.
For instance, when building a structure such as a pier supporting a bridge across a channel on bedrock under water, spaces are created between the underside of the frame of the structure and the bedrock surface due to the roughness of the bedrock surface. Therefore, if underwater concrete such as prepacked concrete is placed in the frame under this condition, mortar will leak out from said spaces to cause the wastage of mortar as well as the contamination of the water. Hitherto, in order to prevent such leakage of mortar, it has been a common practice to lay elastic sponge mats along the outer edges of the base of the frame to seal the spaces.
These sponge mats are usually molded from a foamed plastic such as polyurethane rubber so that the interior of the mat has a coarse foam structure but the skin has a dense foam structure which has a relatively low water permeability. Therefore, if a sponge mat of such a structure is sunk under water along with the frame, since water is unable to penetrate into the mat quickly, the sponge mat is compressed by the water pressure and becomes unable to perform its intended function of sealing the spaces between the base of the frame and the bedrock. In order to solve this problem, it has been proposed to provide a plurality of small holes in the skin of the mat which extend into the inner coarse structure.
The provision of such small holes in the skin of the sponge mat allows the relatively easy penetration of water into the mat. However, when the base of the frame touches the bedrock surface and compresses the sponge mat, since such compression is effected rapidly in a short time and the small holes in the skin are collapsed and blocked, water which has penetrated into the mat can not find its way out and is trapped inside the mat. Consequently, the sponge mat swells out sidewise under the high internal water pressure, and when its deformation exceeds the limits of endurance, the mat bursts. When such damage is extensive, the sponge mat becomes unable to perform its function satisfactorily as a seal. The risk of occurrence of such damage is large when the penetration rate of water into the sponge mat is extraordinarily high as often experienced in construction work in deep water, or when the compressibility of the mat is extremely high because of the enlarged size of the mat.